Method of producing chromates



y 1934- J. B. CARPENTER, JR, ET AL 1,964,719

I METHOD OF PRODUCING CHROMATES Filed July 13, 1932 Zmvemg Emmi j?Sievemsom, Jah a Patented July 3, 1934 UNITED STATES PATENT OFFICE P.Stevenson,

Newton,

Mass., assignors to Arthur D, Little, Incorporated, Cambridge, Mass., acorporation of Massachusetts Application July 13, 1932, Serial No.622,288 I 6 Claims.

This invention relates to the manufacture of sodium chromate. Thecommercial procedure in producing sodium chromate at the present timeinvolves mixing chrome ore, sodium carbonate and lime, spreading themixture on one or more hearths of a reverberatory furnace and roastingit for a prolonged period to effect an oxidation of the chromiumconstituents of the ore from a trivalent to a hexavalent state. Thismode of 10 treatment necessitates laborious and expensive rabbling orturning-over of the mixture in order to obtain suflicient oxidization.

It is the principal object of this invention to provide an economicalprocess for making sodium chromate without the present rabblingoperation and which increases the efliciency of the oxidizing reactioninvolved, and results in the conversion of a high percentage of thechromium in the ore to sodium chromate.

There have been proposed various processes involving the briquetting ofthe ore with sodium carbonate, and with or without other ingredients,and the subsequent roasting of such briquettes. As far as we are aware,these proposals so to form the reacting constituents have failed to comeinto commercial use, for the reason apparently that the proceduresrecommended have entailed greater cost, either by way of increasedoperating expense or decreased chemical efliciency, than the establishedpractice of rabbling in a reverberatory furnace. Our processcontemplates the possibility of the use of low grade chrome ores byvirtue of the fact that the type of operation herein described permitsthe widening of the production range while maintaining efficientoperation of the furnace. The principal chemical reaction involved inthe process is oxidation. In utilizing briquettes of ore and sodiumcarbonate with or without the admixture of lime and/or otheringredients, proposals to compensate for the omission of the rabblingoperation by excluding furnace gases from the bricks, (presumably toincrease the oxygen content of the available air) involve an indi- 45rect heating of the briquettes, a decreased thermal efllciency of theprocess and an increased cost of installation and maintenance of thefurnace. Proposals to lay the briquettes on the hearths of reverberatoryfurnaces and there roast them without rabbling involve lack of. adequateand rapid contact of air with the interior of the mass, with much thesame detrimental effect as omitting the rabbling operation of ordinarycommercial reverberatory furnace roasting. The gas 5: in the zone of thehearth of a reverberatoryfurnace is likely to be comparatively stagnantand not at all disposed to provide a rapid circulation into the interiorof a brick placed on the hearth. Proposals to promote oxidation in theabsence of rabbling by including an oxidizing chemical in the bricks areprohibitively costly, since the oxidizing chemical used will cost morethan the customary rabbling sought to be replaced. The fact that theindustry with which this invention is concerned has not adopted the ideaof roasting the ore in briquetted form reflects failure to conceive ofconditions under which bricks of substantial size can be formed toeffect accessibility of oxidizing gases into the interior and means forroasting to insure a uniform diffusion of oxygen throughout the mass ofa brick. Ou invention is directed to these ends.

The important reaction that takes place when chrome ore is roasted withsodium carbonate may be expressed by the followingequation:

Accordingly, in the presence of sufficient oxygen, one molecule offerrous chromite (chrome ironstone or chrome iron ore) reacts with twomolecules of sodium carbonate to give two molecules of sodium chromateplus carbon dioxide and ferric oxide. Hence, from a given amount ofsodium carbonate only a definite or equivalent amount of sodium chromatecan be produced, providing the reaction is complete. We consider themaximum quantity of sodium chromate that can be theoretically producedfrom a given quantity of sodium carbonate to constitute 100 per cent.conversion, regardless of the amount of chromium present in the oreused, and when we use the term 90% conversion we mean that an amount ofsodium chromate has been formed which is equal to 90% of the theoreticalequivalent of the sodium carbonate used.

We have discovered a procedure wherein briquetting of the roast may bepracticed consistently with lowering the cost of sodium chromateproduction. Ourimproved process involves as an initial step theformation of self-sustaining masses of the material, (for example massesin the shape of oblong bricks or briquettes), and hereinafter forconvenience referred to as bricks. We have discovered that for theeflicient and economical practice of the process the factors involved inthe formation of the bricks should be so regulated as to render thebricks quite permeable to oxidizing gases at temperatures up no to 2000F. and for this purpose to provide in the bricks a certain minimumproportion of voids, as more fully described below. These voidcontainingbricks are then subjected to a roasting process in a furnace wherein theoxygen in the furnace gases comes into direct contact with the bricks.

A preferred form of furnace is 9. cont uous tunnel kiln which permitspiling of the bricks on cars or other conveying means in such manner asto occupy in the furnace the major part of the cross-sectional areaaccessible to the oxygen in the furnace gases. The object is thus toprovide a body of piled-up bricks of porous character through, betweenand close to which, the draft of the furnace carries the oxygen in thefurnace gases, with the consequent attainment and maintenance ofvelocitiesand local pressures of this oxygen suflicient to causeenhanced diffusion of the oxygen through the interior of the porousbricks. Sufficient air can be admitted to the furnace of the kiln tomaintain'an adequate oxidizing condition. The passing of the furnacegases containing suitable oxygen contents through a porous obstructionconstituted of a pile of the porous bricks is considered to be a veryadvantageous feature. The heat of the gas is imparted to the bricksdirectly, as distinguished from the less efficient and prohibitivelycostly'indirect heating action of a muille furnace; the acquisition ofheat by the bricks is accompanied by an abundant forced penetration ofoxygen through the voids of the bricks, as distinguished from theheating of bricks by radiation in the more or less stagnant zone of thehearth of a reverberatory furnace and/or the usual type of mufllefurnace.

A specific instance of our improved process will now be described by wayof example.

In the drawing:

Fig. 1 is a diagrammatic plan view, partly broken away, of adirect-fired tunnel kiln adapted to be employed in our improved process;and

Fig. 2 is a vertical transverse sectional view of the kiln of Fig. 1showing particularly the manner in which the bricks including chrome oreand sodium carbonate are piled so as to be subjected to the direct flowof gas in the tunnel of the kiln.

We have discovered that the efliciency of conversion or percentage ofyield, may be'considerably increased if the bricks are carefully formedso as to have a suflicient porosity to be quite permeable to the hotoxidizing gases to which they are subjected during roasting. This may beexplained by the fact that the principal chemical reaction is one ofoxidation and that the ability of the hot oxidizing gases to enter theinnermost parts of the brick determines to a large extent thecompleteness of the reaction. We have further discovered that if acertain minimum porosity is not attained, ineflicient conversion willresult. We prefer to form the bricks with at least 30% voids as measuredby the volumenometer described on page 169, Vol. I, 1929, of the Journalof Industrial and Engineering Chemistry, this percentage of voidsrelating to the condition of the bricks when they have been air driedprior to roasting; a percentage of voids of 40% is even more preferable.

Conversions of over 95% have been consistently obtained by formingbricks measuring "x2"x3" from a uniform mixture of the followingingredients, no particle of which will fail to pass through an mesh U.S. standard Tyler sieve:

- Per cent on Grams dry basis Chrome ore (48.98% C1'2O3) 608 40.8 Sodaash 380 25.5 Lime (CaO) 500 33.? Water for mixing 300 Percent Chrome ore(48.98% C12O3) 43.5 Soda ash 30.0 Lime, CaO 26.5

when formed in the above manner and roasted for 6 hours at a temperatureof 1900 F. give a conversion of chromite to chromate in excess of eventhough the percentage of lime used is considerably below that foundnecessary for present commercial roasting of chrome ore. We have foundthat bricks of this composition show a lower volatilization loss ofchromium than those made with the initial mixture.

The bricks may, if desired, be formed by an ordinary pressing or moldingoperation in which, in order to obtain the desired porosity, onlysufficient pressure is applied to cause the bricks to maintain theirform when subjected to the usual shocks of handling. As illustrative ofthe advantage of carefully forming the bricks with sufficient voids, wehave found that, with a given mixture of ingredients, a pressure of 350lbs. per square inch will make a brick approximately %"x2x3.sufliciently capable of standing shock provided the molding or formingpressure is released as soon as it reaches 350 lbs. per square inch. Ifall of the other conditions-are maintained identical and the pressure of350 lbs. per square inch is retained for thirty seconds, the bricks thusformed show when roasted a decrease in conversion of as much as 8%(based upon the sodium carbonate content) below the bricks from whichpressure is released immediately upon reaching 350 lbs.

Bricks for use in our improved process may also be made by extrusion.Bricks formed in this manner are in some respects preferable to bricksformed by molding in that the extruded bricks can carry more water andrequire less pressure to hold their shape and hence can be made inlarger sizes and still retain a satisfactory porosity. Bricks have beenextruded in the standard fire brick size (2}5"x4'/2"x8 /2") and roastedwith the resulting conversion of over 90% (based on the sodium carbonatecontent).

The exact pressures to be employed in forming the bricks either bymolding or extrusion, and the amounts of water to be mixed with theingredients in forming the bricks will, of course, de-

1,964,719 pend upon the size of the bricks formed. The

conditions of pressure and water content can best be regulated tocontrol the porosity of the bricks by measuring the percentage of voidsin a dry brick by the volumenometer mentioned above and then decreasingthe pressure or increasing the water content, if necessary, to providethe necessary percentage of voids.

Referring to Figs. 1 and 2, the formed bricks are piled upon cars 11such as are used in continuous tunnel kilns, and the cars then passedcontinuously through a dryer (not shown) at a rate of travel such thatthe moisture is removed sufliciently to prevent undue cracking of thebricks in the subsequent roasting operation. Thereupon the loaded cars11 pass continuously through the kiln 12 from its entrance end 12 to itsexit end 12'. The tunnel kiln may, for example, have adjacent to itsentrance end a preheating zone, may have its hottest zone in the middleof its length in which are located furnace units 13 discharging theirproducts of combustion into the tunnel of the kiln, andmay have anunheated cooling zone located adjacent to the exit end of the tunnel, asindicated in Fig. 1. The tunnel kiln is of the direct-fired type inwhich the flow of the products of combustion of the furnace is in adirection opposite to that of the travel of the bricks, which action isdue to the arrangement of the exhauster 17 which draws the spent gasesthrough the preheating zone and discharges the same adjacent to theentrance 12 and the arrangement of the blower 16 which continuallydischarges a current of air into the cooling zone where it is preheatedby being forced around the bricks discharged from the firing zone. Theair thus admitted not only serves to coolthe bricks, but also insuresthe continued maintenance of a positive oxidizing atmosphere within thefiring zone of the furnace. Such kilns are well known and need not befurther described.

Referring to Fig. 2, the bricks are piled in a more or less open stack15 on the cars 11 in such a way that the stack of bricks thus formedoccupies as much as possible of the cross-sectional area of the tunnel,occupying 'as shown considerably more than a major portion of such crosssection( The effect of this is to confine the stream of furnace gases,including the necessary air for oxidation, largely to flow through thestack of bricks.

The temperature of the hottest zone of the kiln is preferably adjustedto from 1700 F. to 2000 F., although higher or lower temperatures may beemployed if desired. The gas stream flowing through the tunnel of thekiln is adjusted by the admission of sufiicient air to the furnaces ofthe kiln, and if desired, at other points (e. g., the discharge end ofthe furnace), to cause the stream to have an adequate oxidizing action.The

Y within the kiln is to cause an abundant absorption of oxygen into thevoids of the bricks accompanied by a rapid and eflicient transfer ofheat to the bricks. The draft of the tunnel kiln insures rapidcirculation of the hot oxidizing and heating gases, preventingstagnation and providing uniformity of action on all of the brickspassed through the kiln.

The increase in temperature and amount of hot oxidizing gases caused topenetrate the bricks by our improved process renders possible a veryeflicient chemical reaction between the chrome ore and sodium carbonateingredients. The conditions of heat transfer are materially improvedover those prevailing in the usual practice of rabbling the mixture in areverberatory furnace, and over those which would prevail were any ofthe proposed briquette-roasting processes to be attempted. The generaloverall efficiency of our process is such that it can replace thecustomary procedure, with resulting economy in operating cost. It willbe evident that the working conditions prevailing in the practice of ourprocess are greatly improved over all processes involving the rabblingof mixtures in furnaces, since the only handling to which the bricks aresubjected is done wholly outside of the kiln, in stacking and inremoving the bricks from the kiln cars. Whereas in reverberatoryfurnaces, the mixture has had to be laboriously rabbled, and manuallytransferred from one section of the furnace to another, the carscarrying the bricks in our process can be propelled through the kilnwith a minimum of effort, any suitable mechanical car pusher serving toadvance the whole line of cars at desired intervals.

The bricks after having been roasted are then treated by any suitableextraction process, to obtain a solution containing the desired sodiumchromate, and if desired, the residue can be treated as described in ourcopending application Serial No. 622,289, filed concurrently.

It should be understood that the present disclosure is for the purposeof illustration only and that this invention includes all modificationsand equivalents which fall within the scope of the appended claims.

We claim:

1. Process of making chromates by forming bricks including chrome oreand a suitable carbonate and roasting the bricks, characterized in thatthe bricks are sufficiently porous to be permeable to hot gases and arearranged in stacks, and a stream of hot oxidizing combustion gases froma furnace is caused to flow through the stack of bricks, thereby topromote a uniform penetration of heated oxidizing gas into the interiorof the bricks.

2. Process of making chromates by forming bricks including chrome oreand a suitable carbonate and roasting the bricks, characterized in thatthe bricks are stacked and a stream of hot oxidizing combustion gasesfrom a furnace is caused to flow through the stack of bricks, the brickshaving at least approximately 30% of voids, by volume, thereby topromote penetration of heated oxidizing gas into the interior of thebricks.

3. Process of making chromates by forming bricks including chrome oreand a suitable carbonate and roasting the bricks, characterized in thatthe bricks are stacked and a stream of hot oxidizing combustion gasesfrom a furnace is caused to flow through the stack of bricks, the brickshaving at least approximately 40% of voids, by volume, thereby topromote penetration of heated oxidizing gas into the interior of thebricks.

4. Process of making chromates by forming bricks including chrome oreand an alkali-metal carbonate and roasting the bricks, including movingthe bricks through the preheating, hottest and cooling zones of adirect-fired continuous kiln having a flow of hot oxidizing combustiongases, the bricks being sumciently porous to be permeable to hot gasesand being in the form of a stack occupying the major part of thecross-section of the kiln, whereby to constrain the stream of hot gasesin the kiln to flow through the stack and penetrate the bricks.

5. Process of making sodium chromate which comprises forming brickswhich include chrome ore and sodium carbonate and contain at least 30%voids, by volume, and subjecting the bricks to a roasting operation,including moving the bricks through the preheating, hottest and coolingzones of a direct-fired continuous kiln having

